1. Field of the Invention
The present invention relates to wireless communication, such as cellular networks. In particular, the present invention relates to the structure and allocation of resources in wireless communication systems, such as cellular communication systems using orthogonal frequency division multiple-access (OFDMA) signaling protocols.
2. Discussion of the Related Art
Because of its better resource resolution in the time-frequency grid, OFDMA has attracted significant attention and has been adopted in many wireless systems. The better resource resolution also provides better exploitable diversity when proper resource allocation and resource structuring schemes are adopted. Resource allocation or sub-carrier and power allocation for OFDMA systems are discussed, for example, in (i) U.S. Patent Application Publication 2005/0265223, entitled “Method and apparatus for scheduling downlink channels in an orthogonal frequency division multiple access system and a system using the same,” by J.-H. Song, filed on May 13, 2005 and published on Dec. 1, 2005; (ii) U.S. Patent Application Publication 2006/0109865, entitled “Method for allocating resources in a multicarrier system and transmission apparatus using the same,” by S.-Y. Park, Y.-W. Lee, S.-B. Yun, Y.-S. Kim, filed on Nov. 25, 2005 and published on May 25, 2006; (iii) U.S. Pat. No. 6,917,812, entitled “Air interface scheduler for wireless communication networks,” by A. Damnjanovic, filed on Dec. 3, 2001, and issued on Jul. 12, 2005; (iv) U.S. Pat. No. 6,987,738, entitled “Method for packet scheduling and radio resource allocation in a wireless communication system,” by V. G Subramanian, R. Agarwal, R. J. La, filed on Jan. 12, 2001 and issued on Jan. 17, 2006; (v) U.S. Patent Application Publication 2006/0149620, entitled “Scheduling apparatus and method in a multicarrier communication system,” by C.-H. Suh, S.-H. Park, S.-H. Yoon, S.-K. Hong, Y.-K. Cho, filed on Dec. 30, 2001 and published on Jul. 6, 2006; (vi) U.S. Patent Application Publication 2008/0076438, entitled “Method for dynamic resource allocation of uplink and downlink in OFDMA/TDD cellular system,” by K. H. Chang, S. J. Ko, T. H. Sun, J. H. Kim, filed on Sep. 21, 2007 and published on Mar. 27, 2008; and (vii) U.S. Patent Application Publication 2008/0043610 (“Liu”), entitled “Multi-carrier communications with group-based subcarrier allocation,” by X. Li, H. Liu, H. Yin, G. Xing, E Mu, filed on Oct. 26, 2007 and published on Feb. 21, 2008. For example, Liu discloses a method for subcarrier selection in an OFDMA system, which partitions subcarriers into groups of one or more clusters of subcarriers for use in communication with the subscriber1. These resource allocation schemes in the prior art neither exploit multiuser diversity benefits nor take into account a time-varying channel. 1Subscriber is also known as mobile station (MS) or user equipment (UE).
Some prior art schemes exploit multiuser diversity using complete or partial knowledge of the channel to guide resource allocation or scheduling. Such schemes includes (i) the article, entitled “Opportunistic beamforming and scheduling for OFDMA systems,” by P. Svedman, S. K. Wilson, L. J. Cimini, and B. Ottersten, published in IEEE Trans. Commun., vol. 55, no. 5, May 2007, pp. 941-952; (ii) the article (“Chemaly”), entitled “Adaptive resource allocation for multiuser MIMO/OFDM networks based on partial channel state information,” by R. Chemaly, K. Letaief, and D. Zeghlache, Proc. IEEE GLOBECOM 2005, St. Louis, Mo., November 2005, pp. 3922-3926; (iii) U.S. Patent Application Publication 2007/0110003 (“Tujkovic I”), entitled “Subcarrier allocation in OFDMA with imperfect channel state information at the transmitter,” by D. Tujkovic, A. Paulraj, filed on Jun. 16, 2006 and published on May 17, 2007; and (iv) U.S. Patent Application Publication (“Tujovic II”), entitled “Time domain interference averaging with multiuser diversity in OFDMA systems,” by D. Tujkovic, A. Paulraj, filed on Jun. 16, 2006 and published on Jan. 3, 2008.
Such channel knowledge-based schemes are mostly applicable only to quasi-static or slow-varying channels. For example, Tujovic I fails to consider the impact of time-varying channels which deteriorate multiuser diversity gain. Tujovic I discloses a method that combines features of multiuser diversity and frequency diversity allocation schemes. The method of Tujovic I retains advantages of multiuser diversity allocation whenever possible by assigning a fraction of the available bandwidth to users in high signal-to-noise ratio (SNR) channels. Recognizing that channel state information (CSI) at the transmitter is not perfect, the system and method allocate the remaining bandwidth pseudo-randomly according to frequency diversity. Similarly, Tujovic II reduces interference between multiple users operating under multiuser diversity within a coherence bandwidth in an OFDMA system by spreading out the users' transmission symbols randomly in time within the coherence bandwidth. When transmission symbols are randomly dispersed, the variance of interference between users in the same sub-band is reduced.
Other schemes that incorporate time-variation or Doppler spread in their resource allocation include Chemaly (above) and the articles (i) “Sub-band rate and power control for wireless OFDM systems,” by J. Oh and J. M. Cioffi, published in Proc. IEEE Vehicular Technology Conference (VTC 2004-Fall), Los Angeles, Calif., Vol. 3, 26-29 Sep. 2004, pp. 2011-2014; and (ii) “Fair adaptive radio resource allocation of mobile OFDMA,” by F.-S. Chu and K.-C. Chen, published in Proc. IEEE Personal, Indoor and Mobile Radio Communications (PIMRC 2006), Helsinki, Finland, September 2006, pp. 1-5. These time-variation or Doppler spread resource allocation schemes, however, require knowledge of the Doppler spread. Even with knowledge of Doppler spread, these methods still suffer from performance degradation at high mobile speeds. The mismatches between the models used in these methods and the actual channel time-correlation and the maximum Doppler shift may introduce additional degradation.
In the prior art, several resource structures or user channelization exploit diversity, such as (i) band division multiple access (BDMA) in the article “BDMA testbed-configuration and performance results,” by T. Kunihiro, T. Yamaura, M. Suzuki, E. Fujita, published in Proc. IEEE Vehicular Technology Conference, VTC Spring, Vol. 3, 16-20 May 1999, pp. 1836-1840; (ii) interleaved frequency division multiple access (IFDMA), adaptive FDMA (AFDMA), and adaptive block division multiple access (ABDMA) in the article “Resource allocation and power control in a TDD OFDM-based system for 4G cellular networks,” by P. Bisaglia, S. Pupolin, D. Veronesi, M. Gobbi, published in Proc. IEEE Vehicular Technology Conference, VTC Spring, Vol. 4, 7-10 May 2006, pp. 1595-1599; and (iii) non frequency scattering and hopping (NFSH), frequency scattering (FS), and frequency scattering and hopping (FSH) in the article “Performances of multicarrier system with time and frequency domain spreading for wireless communications,” by Y. Teng, K. Naito, K. Mori, H. Kobayashi, published in Proc. International Conference on Wireless Networks, Communications and Mobile Computing, Vol. 1, 13-16 Jun. 2005, pp. 558-563.
Other examples of resource structures include (i) in WiMAX and IEEE 802.16e, distributed permutation and contiguous/adjacent permutation (see, Broadband Wireless Access: IEEE MAN standard, IEEE LAN/MAN Standards Committee IEEE 802.16e, 2005.); and (ii) in 3GPP LTE, localized and distributed type resource blocks (see, 3rd Generation Partnership Project (3GPP); Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8), Doc. 3GPP TR 36.211, Version 8.4.0, September 2008).
All of the above user resource structures may be classified into two main categories: band-type (also known as localized-structure) and interleaved-type (also known as distributed-structure). In a band-type resource structure, a user is allocated a band of adjacently located subcarriers. In an interleaved-type resource structure, a user is allocated subcarriers that are (i) spread out across the entire band in use, (ii) interleaved with those of the other users, or (iii) both. In such a user resource structure, the subcarriers may be non-contiguous, or may be provided in several non-adjacent groups of contiguous subcarriers. The assigned subcarriers may also change from symbol to symbol in a systematic way across all users. Each of these user resource structures have their own advantages as well as disadvantages, depending upon application, usage, channel condition, mobility and other factors.
In an OFDMA system, multiuser diversity may be exploited by assigning to each user contiguous subcarriers (i.e., a band-type user resource structure). If a channel varies substantially within a transmission frame, a centralized OFDMA resource allocation scheme may lose most of its multiuser diversity gains as a result of the outdated or mismatched channel information used in the resource allocation at the beginning of the frame. In fact, the channel allocated to a user may well be in deep fade at a later part of the frame. In order to alleviate the effect of such an adverse deep fade, one may provide a shorter frame length, or a more frequent update (feedback) of channel information, at the cost of substantial throughput loss due to large overhead (e.g., preamble and control information, such as DL-MAP and UL-MAP). A very short frame length is therefore overhead inefficient. Alternatively, the time-varying channel effect may also be alleviated by keeping the subcarriers assigned to a user spread out over the entire band (i.e., interleaved-type). One example of an interleaved-type resource structure is provided in the article “A new ranging method for OFDMA systems,” by X. Fu, Y. Li, and H. Minn, published in IEEE Trans. on Wireless Commun., vol. 6, no. 2, February 2007, pp. 659-669. While an interleaved-type resource structure addresses the deep fade problem through frequency diversity, multiuser diversity gain is lost. Thus, existing user resource structures (i.e., both band-type and interleaved-type) have limited diversity exploitation capability in time-varying channels.